1. Technical Field of the Invention
The present invention relates to analog integrated circuits, and, more particularly, to an integrated circuit which provides a selectable potential between two external terminals.
2. Description of Related Art
Traditional potentiometers are mechanical devices whose resistance varies according to a selected physical position of a wiper, which produces selected electrical potentials. Typically, wiper location is controlled by rotating or sliding the wiper to the desired position. The increased use of integrated circuits and other semiconductor devices includes efforts to develop solid state potentiometers. For example, Xicor's X9MME, discussed in U.S. Pat. No. 5,243,535, a resistor array composed of 99 resistive elements is disclosed wherein the wiper position is digitally controllable. Another example is provided in Banezhad and Gregorian, "A Programmable Gain/Loss Circuit," 22 IEEE Journal of Solid-State Circuits 1082 (1987), a programmable circuit which provides gain/loss values ranging from -25.5dB to +25.5dB in fixed 0.1dB increments. Other devices, perhaps more properly named attenuators, have been developed to provide nonlinear steps to simulate a rising or falling of a resistance value.
One notable aspect of the design in these references, is that they utilize conventional integrated circuit manufacturing techniques. With respect to resistor arrays, it is known that resistance matching between two discrete devices is best accomplished, at the design stage, by designing and building cells of resistors having resistors using equal values and/or equal numbers of resistors. In order to obtain a target resistance, the interconnections between cells can be varied to create the target resistance.
Thus, a desired resistance for a resistance cell of a digital potentiometer is only obtained through random good fortune, especially for lower values of resistance.
Ordinarily, the resulting resistance is only within a variable tolerance range of the desired resistance. For example, in prior art devices a typical tolerance can be in the range of .+-.15% but match to each other with much tighter tolerances, as high as fractions of a percent. Thus, the traditional design approaches to developing these resistor banks and/or digital potentiometers lend themselves to repetitively yielding high levels of precision only on exactly repeated resistances. However, it is generally known that even the smallest steps (dB) in attenuators do not provide the required resolution to mimic a smoothly changing resistance needed to produce nonlinear steps. These attenuators cause or introduce gain distortions that are only marginally tolerable at audio frequencies. These distortions become unsuitable when precise resistances are required, especially at low values of resistance such as on the order of below 100 ohms. The prior art, does not, therefore, appear to provide a solution for a digital potentiometer requiring precise, small, logarithmic steps.
In addition to not providing precise resistance values, digital potentiometers have other shortcomings. First, at the front end, most of the prior art digital potentiometers do not have the capability of directly connecting to a switch on a panel, for example of a stereo. While mechanical potentiometers are generally mounted directly to a stereo panel, it is common to interface a digital potentiometer to a switch or switches through a microprocessor which may be under software control. Using this approach, when a volume increase is selected, digital potentiometers may frequently introduce detectable noise, especially in the form of clicks into the signal as different wiper points are selected while a given signal is being amplified.
The problem of clicking sounds resulting from noise being introduced or added to the signal being amplified has been addressed by several prior art references. Some have attempted to solve the problem by providing an active filter to "purify" the signal. Other approaches have also been suggested depending upon the perceived problem.
However the problem which has not been identified in the prior art is that the change from one wiper point to another, in a digital potentiometer, occurs while a given signal is being amplified. Thus, there is a need for a circuit for controlling and effecting any changes in wiper positions only when there is no input signal, or when the input terminals are equal; a circuit which synchronizes wiper position changes to correspond with a crossing of the signal which is being amplified solves the prior art problems.
Digital potentiometers which may be directly attached to a switch as a mechanical potentiometer, which reduce the noise added to a signal during changes in amplification, which provide accurate and predictable attenuation in logarithmic increments to account for human hearing sensitivity, and which consume less power to operate are needed.